A multi-frequency transducer is a type of transducer which is operated on a frequency selected in accordance with targets of search. This type of transducer is used in underwater sounding apparatus, such echo-sounders and sonars. A typical example of the multi-frequency transducer is a two-frequency transducer which is designed to transmit and receive ultrasonic waves at either of two frequencies. Although the two-frequency transducer of this kind may be provided with two transducer elements designed to operate independently of each other on two frequencies, this approach results in an increase in the overall physical size and cost of the transducer. In this situation, a transducer capable of transmitting and receiving ultrasonic waves at two frequencies with a single transducer element is currently available for practical use.
A transducer 100 shown in FIG. 11 has conventionally been used as the aforementioned kind of transducer. The transducer 100 is constructed of a piezoelectric ceramic material (PZT), for example, which is shaped into a circular disc having a diameter L1 and a thickness L2. This transducer 100 has a natural resonant frequency f1 determined by the value L1 as well as a natural resonant frequency f2 determined by the value L2, wherein there is a relationship f1<f2 between the two resonant frequencies because L1>L2. Therefore, if a transmit signal of which frequency is equal to one of the two resonant frequencies f1, f2 is introduced into the transducer 100, it resonates at the applied frequency and radiates ultrasonic waves at the frequency f1 or f2 whichever applied. When receiving echo signals reflected back from underwater objects, the transducer 100 resonates again at the frequency f1 or f2, whichever applied, and produces a receive signal at that frequency. It is possible to transmit and receive ultrasonic waves at the two frequencies with this single-element transducer 100.
However, the transducer 100 has only two distinct dimensions, L1 and L2, since it is shaped into a circular disc. The values L1 and L2 are uniquely determined when the resonant frequencies are determined. On the other hand, the aforementioned disc-shaped transducer 100 normally radiates ultrasonic waves from its circular surfaces, so that the angle of directivity of the transducer 100 is determined by its dimension L1. Since the values L1 and L2 are determined by the resonant frequencies, it is impossible to freely select the angle of directivity as a function of the value L1. Thus, the dimensions and the angle of directivity of the conventional transducer are not mutually independent. Since the angle of directivity is automatically determined when the resonant frequency is determined, it has been impossible to obtain a desired angle of directivity.
This invention has been made in light of the aforementioned problems. Accordingly, it is an object of the invention to provide a multi-frequency transducer of which dimensions and angle of directivity are made mutually independent so that a desired angle of directivity can be obtained regardless of frequencies.